The present invention relates to earth-penetrating drill bits, and particularly to pressure compensation systems in so-called roller-cone bits.
1. Background
Rotary Drilling
Oil wells and gas wells are drilled by a process of rotary drilling, using a drill rig such as is shown in FIG. 3. In conventional vertical drilling, a drill bit 110 is mounted on the end of a drill string 112 (drill pipe plus drill collars), which may be several miles long, while at the surface a rotary drive (not shown) turns the drill string, including the bit at the bottom of the hole.
Two main types of drill bits are in use, one being the roller cone bit, an example of which is seen in FIG. 2. In this bit a set of cones 116 (two are visible) having teeth or cutting inserts 118 are arranged on rugged bearings. As the drill bit rotates, the roller cones roll on the bottom of the hole. The weight-on-bit forces the downward pointing teeth of the rotating cones into the formation being drilled, applying a compressive stress which exceeds the yield stress of the formation, and thus inducing fractures. The resulting fragments are flushed away from the cutting face by a high flow of drilling fluid.
The drill string typically rotates at 150 rpm or so, and sometimes as high as 1000 rpm if a downhole motor is used, while the roller cones themselves typically rotate at a slightly higher rate. At this speed the roller cone bearings must each carry a very bumpy load which averages a few tens of thousands of pounds, with the instantaneous peak forces on the bearings several times larger than the average forces. This is a demanding task.
2. Background
Bearing Seals
In most applications where bearings are used, some type of seal, such as an elastomeric seal, is interposed between the bearings and the outside environment to keep lubricant around the bearings and to keep contamination out. In a rotary seal, where one surface rotates around another, some special considerations are important in the design of both the seal itself and the gland into which it is seated.
The special demands of sealing the bearings of roller cone bits are particularly difficult. The drill bit is operating in an environment where the turbulent flow of drilling fluid, which is loaded with particulates of crushed rock, is being driven by hundreds of pump horsepower. The flow of mud from the drill string may also carry entrained abrasive fines. The mechanical structure around the seal is normally designed to limit direct impingement of high-velocity fluid flows on the seal itself, but some abrasive particulates will inevitably migrate into the seal location. Moreover, the fluctuating pressures near the bottomhole surface mean that the seal in use will see forces from pressure variations which tend to move it back and forth along the sealing surfaces. Such longitudinal xe2x80x9cworkingxe2x80x9d of the seal can be disastrous in this context, since abrasive particles can thereby migrate into close contact with the seal, where they will rapidly destroy it.
Commonly-owned U.S. application Ser. No. 09/259,851, filed Mar. 1, 1999 and now issued as Ser. No. 6,279,671 (Roller Cone Bit With Improved Seal Gland Design, Panigrahi et al.), copending (through continuing application Ser. No. 09/942,270 filed Aug. 27, 2001 and hereby incorporated by reference) with the present application, described a rock bit sealing system in which the gland cross-section includes chamfers which increase the pressure on the seal whenever it moves in response to pressure differentials. This helps to keep the seal from losing its xe2x80x9cgripxe2x80x9d on the static surface, i.e. from beginning circumferential motion with respect to the static surface. FIG. 4 shows a sectional view of a cone according to this application; cone 116 is mounted, through rotary bearings 12, to a spindle 117 which extends from the arm 46 seen in FIG. 1. A seal 20, housed in a gland 22 which is milled out of the cone, glides along the smooth surface of spindle 117 to exclude the ambient mud 21 from the bearings 12. (Also visible in this Figure is the borehole; as the cones 116 rotate under load, they erode the rock at the cutting face 25, to thereby extend the generally-cylindrical walls 25 of the borehole being drilled.) The present application discloses a different sealing structure, in place of the seal 20 and gland 22, but FIG. 4 gives a view of the different conventional structures which the seal protects and works with.
A critical part of the design of a xe2x80x9croller conexe2x80x9d drill bit is the sealing system. The roller cone bit, unlike any fixed-cutter bit, requires its xe2x80x9cconesxe2x80x9d to rotate under heavy load on their bearings; when the bearings fail, the bit has failed. The drilling fluid which surrounds the operating bit is loaded with fragments of crushed rock, and will rapidly destroy the bearings if it reaches them. Thus it is essential to exclude the drilling fluid from the bearings.
Rock bit seals are exposed to a tremendously challenging fluid environment, in which large amounts of abrasive rock particles and fines are entrained in the fluid near one side of the seal. Moreover, the very high-velocity turbulent flows cause fluctuating pressures near the seals.
Fluid seals are therefore an essential part of the design of most roller-cone bits. However, an important aspect of seal functioning is control of differential pressures; if the pressure inside the seal becomes substantially less than the pressure outside the seal, particulates from the drilling fluid can be pushed into or past the dynamic face. (This can lead to rapid destruction of the seal.) A pressure compensation arrangement is therefore normally used to equalize these pressures.
The life of a rotary-cone drill bit is usually limited by bearing failure, and that in turn is heavily dependent on proper sealing and lubrication. Such bits usually include a grease reservoir in each arm, connected to supply grease to that arm""s bearings. Since the bearing will operate at low speeds, high load, and fairly high temperature (possibly 250xc2x0 F. or higher), the grease used is typically quite stiff at room temperature. However, to provide pressure equalization between the reservoir and the bearings, it is desirable to avoid air pockets in the grease.
When the grease reservoir is filled at the factory, a vacuum is usually applied to remove trapped air, and then the grease is injected under some pressure (e.g. 2000 psi or so). The reservoir""s pressure-relief valve operates to limit the pressure inside the reservoir to an acceptable level, but this still implies a positive pressure which slightly distends the reservoir""s elastomeric diaphragm.
With the old hydrodynamic seals, where some grease leakage past the seal was intentionally designed in, depletion of the reservoir during the service lifetime was a major concern. However, this is not much of a concern anymore. Thus the main purposes of the reservoir now are to assist in complete filling of the bearing and passageways, and to provide pressure compensation in-service.
The normal pressure compensation arrangement uses a tough concave diaphragm to transmit the pressure variations from the neighborhood of the cones to the bearings. The diaphragm is typically filled with grease, and is fluidly connected (on its concave side) through a grease-filled passageway to the grease volume inside the seal. The exterior of the diaphragm is fluidly connected, through a weep hole, to the volume of drilling fluid below the bit body.
One current production system uses a pierced rubber plug (which is separate from the diaphragm) for pressure relief. However, since the phase of pressure transient waves at this plug will not precisely match with those at the diaphragm, this can result in underprotection or overprotection by the plug (i.e. insufficient OR excessive extrusion of grease). Moreover, it was found that the frequent transients seen at the plug would fatigue it.
Pressure Relief System
The present application discloses roller-cone-type bits and methods where a modified pressure compensation structure is used to keep the pressure differential across the dynamic rotary seal within a predetermined operating range. In various embodiments, the pressure relief valve is either made integral with (or very closely coupled to) the lubricant reservoir""s diaphragm. Thus there is little or no phase shift between the diaphragm and the pressure relief valve, and overpressures are accurately limited. Preferably this is achieved by using a hydrostatically-asymmetric seal, which is integrated with or in proximity to the diaphragm, as the pressure relief valve.
In one class of embodiments, the lip of the concave diaphragm is turned back to make a seal which faces in the desired direction. (That is, the direction of lubricant flow into the concavity is the same as the xe2x80x9ceasyxe2x80x9d direction of lubricant flow past the seal.) This choice is somewhat surprising, since it requires some care in the assembly operation (and appropriate chamfering to not tear the seal edge during assembly); but this turned-back lip provides several advantages. First, the overpressure bypass path is very close to the interior of the diaphragm. Second, the overpressure bypass path is short. Third, when vacuum is applied before grease is injected, the preferred lip seal will hold vacuum for the necessary time. Fourth, this orientation permits an overall reservoir design which is very compatible with existing bit designs. Fifth, the overall piece count is not increased.
Thus one advantage of the hydrostatically-asymmetric-seal pressure relief is its close proximity to the diaphragm.
Another advantage is the relatively low fluid impedance of the seal once fluid bypass flow begins.
Another advantage is simple manufacturing.